Building engines can sometimes be a guessing game. No matter how much experience you have or how many different combinations you've already tested, there's always some guesswork involved in putting together the next package. Of course, you want every engine you build to be the best, and it's difficult to shy away from a proven combination if there's only a slim chance of making a few extra ponies with an untested engine. Failure always looms on the horizon, and when it's your hard-earned dollars on the line, accepting defeat is hard to do. That's why SUPER CHEVY is proud to offer you perfection. In its third installment our 408-cid Rat has reached nirvana. According to the theories and formulas offered to us by engine building wizards far superior in knowledge and wit, no more power can be made with our current combination. That means another simple cam and intake manifold swap would not do the trick. To pound that point further into our psyche we tried a bigger cam, and it didn't help. If more power is to come later, it would have to come from better breathing cylinder heads, or so we've learned.
You'd never think that it would be so easy to make 575hp power on pump gas, especially if you do it with off-the-shelf parts. Back in the Feb. issue we showed you how to turn a long-forgotten 396-cid big-block into a 500hp pump gas bruiser. Then, in the Apr. issue we took it to the next level by installing a bigger cam and slipping a trick spacer between the Road Demon 750 carb and the new Edelbrock Victor Jr. intake manifold we bolted on and were rewarded with more power. This pump gas Rat now had no trouble making 542 hp at 6,500 rpm and cranked out 501 lb-ft of torque at 5,000 rpm. This month we figured we might make 600 hp with a little help from some negative crankcase pressure. To keep everything in perspective, we applied the same rules to this build-up that we've adhered to throughout the series, so that anyone could duplicate our figures if they follow our recipe. That meant that every part we used had to be right out of the box with no tweaking/porting/massaging of any kind allowed.
And it still had to live on 92 octane, 76 Performance unleaded pump gas. We trucked the baby Rat back to Westech Performance and bolted it onto their Superflow 901 dyno for another series of tests. After a baseline was established with the old combination, we bolted on a Race Demon RS 950 carb with annular boosters. Out-of-the-box power with the bigger carb did not excite us, but after some jet and air bleed changes and with a new Wilson 1 3/4-inch open spacer underneath the new carb, the engine began to respond. It was not until we swapped rocker arm ratios did we see a notable improvement in power.
The 1.8:1 ratio rocker arms combined with the new carb and spacer put us close to 560 hp. Next we felt we might be trying to push too much air through the small 1 3/4-inch headers we'd used so far, so we installed a pair of 2-inch Hedman pipes with new Borla 3 1/2-inch XR-1 mufflers and got a minor power gain in return.
Finally, in a last ditch attempt to gain every ounce of power--after changing to a light-weight synthetic motor oil and even adding Prolong Engine Oil Treatment--we plumbed a "borrowed" Moroso vacuum pump to the motor. Although this engine was not built for optimum power with a vacuum pump, it was still worth a few extra ponies with negative pressure in the crankcase. We've seen race engines gain over 30 hp when a vacuum pump is connected, but our street Rat only gained 13. That was hardly a disappointment, however, and it pushed us over the 575hp mark. A few more pulls were made to optimize the package further, and we found a new power peak of 577 hp. Not quite the 600 hp we were hoping for, so we called SuperFlow to get their suggestions on where to go next. SuperFlow asked us to supply our cylinder head flow figures, which we did, and they quickly ran through some calculations. They reported back to us that there was no more power to be made until we got more air through the heads. They then congratulated us on building the perfect 1.41-hp-per-cid street beast and encouraged us to port the heads a little to get over the 600hp hump. We'll see if we can pull it off next time.
The following graph represents the best power pull with our newly equipped baby Rat. Since the engine kept making more and more power the faster we spun it, we limited this series of tests between 4,000 and 7,000 rpm. We made one pull down to 3,000 rpm and were still rewarded with over 400 lb-ft of torque meaning this engine would have no trouble frying any set of street tires.
Peak HP = 577 @ 6,600
Peak TQ = 504 @ 5,000*
Peak Volumetric Efficiency = 106.4% @ 6,200
HP-per-cid = 1.41
01. We've been swapping cams in and out of this Rat for months now and we finally did some
02. We were curious what kind of cranking pressure this engine would make and found 165 ps
03. We felt we were nearing the power limits of the Speed Demon carb we had been running,
04. Since we were getting closer and closer to the end of the test series for this engine,
05. Since testing began, we've been spinning the engine faster and faster, and it has kept
06. This engine has found its favorite ignition advance and doesn't like anything else. Bu
07. Like most engines, the baby Rat responded favorably to a rocker arm ratio increase. We
08. To get all the power we could out of this engine and show you that vacuum pumps are no
09. Since the vacuum pump and its connecting hoses were plumbed for a different engine, we
10. We looked for every last ounce of power and poured in a bottle of Prolong Engine Oil t
11. We drilled and tapped one hole up high in the driver's side head (hidden behind AN cap
12. Vacuum was monitored and controlled in the passenger side valve cover with a McCLintic
BACK TO THE OLD
It's been several months since the first story in this series ran, and this month we've included a complete listing of all the parts used in the engine and the power they made up to this point. That way you can choose your power level and purchase the parts needed depending on your budget and how fast you want to go. Although this engine still hasn't topped the $8,000 mark to build, the parts we installed to make 575 hp do tack on quite a few dollars to the final tally.
PARTS FOR 500 hp
•396-cid, two-bolt block, bored 0.060 over
•10.25:1 compression ratio
•GM-forged crank, 3.76-inch stroke
•GM truck connecting rods with 3/8-inch ARP Wav-lok bolts (polished and shot-peened)
•Speed Pro engine bearings
•Speed Pro forged pistons PN L2287F
•Total Seal gapless moly-coated rings
•Fel Pro gaskets (PN 1027 head gasket)
•Brodix -1 OEFI aluminum heads with 2.25-1.88 valves
•Comp Cams valve springs PN 939
•Comp Cams locks, pushrods, and Titanium retainers
•Comp Cams 1.7:1 aluminum roller rockers
•Comp Cams XR274R Roller camshaft
•Comp Cams Endur-X roller lifters
•Edelbrock Performer RPM manifold
•Speed Demon 750 carb
•Hooker 1 3/4-inch headers
PARTS FOR 540 hp
•Same as for 500hp except:
•Comp Cams XR280R Roller camshaft
•Edelbrock Victor Jr. intake manifold
•Wilson 1.5-inch four-into-one tapered hole carb spacer
PARTS FOR 575 hp
•Same as for 540 hp except:
•Comp Cams 1.8:1 stainless rocker arms
•Race Demon 950-cfm carb
•Wilson 1 3/4-inch open spacer
•Headman 2-inch headers
•XR-1 3 1/2-inch Borla mufflers
•Moroso three-vane vacuum pump
&bullComp Cams valve springs PN 954
Fire in the Hole
After spending three consecutive days tuning on the dyno we were able to create as close to a perfect fuel curve as possible. Most of that was accomplished by carefully monitoring the Lambda air/fuel ratio and Brake Specific Fuel Consumption (BSFC) figures the Superflow dyno provided. Very little of it was accomplished with the old tuner's trick of reading spark plugs and here's why: When the engine is jetted correctly, spark plugs will show little or no color at all on the visible portion of the porcelain. The coloring will occur deep inside the threaded portion of the plug where the porcelain meets the metal. This is only visible with one of those lighted magnifying scopes that you see used in the pro pits. The "Fire Ring," as its commonly called (dark ring, see arrow, visible on the porcelain), will show the plugs' true color. We cut the threads off this plug to show you what the ring looks like. The light tan shade on the porcelain near the center electrode is a result of dye in the fuel and does not indicate a proper fuel mixture.